In in-line rolling mills, high speeds of operation are essential if undue cooling of the steel below optimal rolling temperatures is to be avoided. Consequently, in such mills, control of both internal microstructure and of external surface properties is difficult to achieve to produce optimum metallurgical characteristics in the finished steel product.
In a conventional continuous casting steel mill using a reversing rolling mill such as a Steckel mill for rolling, steel is cast into a strand by a caster, is severed into slabs or other preferred portions downstream of the caster, and passes to a reheat furnace where it is heated to a uniform pre-rolling temperature. From the reheat furnace, the steel may be rolled by an initial roughing mill, but at least the final rolling steps are effected by a Steckel mill. If thicker plate product is produced by the Steckel mill, the Steckel mill rolling is confined to flat-pass rolling. For thinner, coilable plate and strip product the steel may be coiled within the coiler furnace during at least the later rolling passes. Following rolling, the steel is typically cooled, and if finished as a plate product, is conventionally passed through a hot leveller or cold leveller or both. Coilable products are typically up-coiled or down-coiled and offloaded for shipment; flat plate is cut to preferred length and may be subjected to final cooling on a cooling bed before stacking and shipment.
The use of a Steckel mill to roll steel is well established in industrial practice and in the technical literature. However, the optimization of steel quality from both a surface standpoint and an internal microstructure standpoint, especially for flat steel plate products, has not been achieved by others. In particular, it has not been understood prior to the development of the present invention by the present inventors that a unique combination of steel cooling, steel heating and steel reduction steps, taken in a controlled manner between the caster and the end of the downstream line, can lead to preferred surface and metallurgical characteristics in finished steel plate.